The REUSE helper tools are some helper tools to check and generated tool supported the compliance with the REUSE Initiative specifications. That are standards to make it legally simple to reuse your written software (like correct licence and copyright header).
It would be nice, if Gentoo provide this tool directly in it's packages.
there are ebuilds in overlays.
In the meantime reuse became part of the official tree in all major distributions.
Does anyone want to move it to the official tree?